[virtmagma] correctly define GPU path in BUILD.gn

This change corrects the parent commit (1cfafcad) to pass the GPU path
override macro to anv_device.c when compiling for the linux toolchain.

Test: ran (pending commit) vulkan_unit_tests in biscotti guest
Change-Id: I038e1d35e12e542071c31458da77968ab5105e26
diff --git a/src/intel/vulkan/BUILD.gn b/src/intel/vulkan/BUILD.gn
index 23f902b..8f190ae 100644
--- a/src/intel/vulkan/BUILD.gn
+++ b/src/intel/vulkan/BUILD.gn
@@ -56,9 +56,6 @@
     "//garnet/lib/magma/src/libmagma_linux",
   ]
 
-  # TODO(MA-588): support flexible device path
-  defines = [ "DEV_GPU_PATH_OVERRIDE=\"/dev/wl0\"" ]
-
   ldflags = [ "-static-libstdc++" ]
 }
 
@@ -71,6 +68,11 @@
   ]
   cflags_cc = [ "-Wno-c++11-narrowing" ]
 
+  if (is_linux) {
+    # TODO(MA-588): support flexible device path
+    defines = [ "DEV_GPU_PATH_OVERRIDE=\"/dev/wl0\"" ]
+  }
+
   public_configs = [ ":vulkan_internal_config" ]
 
   deps = [